Air Detectors

Keith Shuttleworth & Associates have extensive experience of designing, developing, installing and validating air detectors on a wide range of sterilization equipment. We have worked with autoclave manufacturers to integrate the devices into existing control system and have developed stand alone models, which simply “bolt on” to existing autoclaves.

What is an air detector?

An air detector is a device commonly fitted to autoclaves processing equipment and porous loads in the UK. Its purpose is to automatically check the process at the end of the air removal stage, and before the heating stage, to ensure that an adequate amount of air has been removed to prevent the sterilization being affected. In the event that it detects too much air, the device is normally configured to cause a cycle to automatically fail and the process to proceed to the drying stage.

The air detector may be considered to be an automatic device that operates during each cycle of operation that will provide additional assurance that an excess of air (and/or non-condensable gases) does not remain at the end of the air removal stage in sufficient quantities to affect the sterilization process.

Sensitivity

Typically, the air detector, when correctly set up will cause a process to fail if sufficient residual air is present at the end of the air removal stage to cause a Bowie Dick test failure (Daily Air Removal Test).

While not specifically designed to detect specific levels of non-condensable gases, in many cases the air detector will sense these, provided they are present in sufficient quantity at the right part of the process.

Regulatory

EN 285 indicates that an air detector is required, though states that a comparison of pressure/temperature can be made using steam tables in lieu of an air detector being fitted. It will be seen below that this approach does not provide sufficient sensitivity, other than in the case of a gross leak (like the autoclave door being left open!). This document provides a method of testing the set up of the air detector.

HTM 2010 Part 3 (replaced by CFPP 01-01) Verification and Testing also provides data on testing air detectors. Unlike EN 285, this document provides detailed guidance on the methodology of setting up an air detector.

Some regulatory agencies will allow a less demanding validation regime where air detectors are installed.

Design/Types

The air detector is usually designed to operate on either temperature or pressure or pressure and temperature.

The temperature controlled design is usually a temperature sensor located at the end of a closed tube connected either to the autoclave chamber or the autoclave drain. The temperature is sensed at the end of the air removal stage and when the chamber pressure is positive (and no inwards leaks can occur). It is usually measured at a specified chamber pressure as determined either by the pressure transducer on the process controller, an independent pressure transducer or a pressure switch. In the event that a surplus of air is present it will accumulate at the end of the tube causing the temperature to be depressed and cause a process failure. If little or no air is present steam will be present at the end of the tube, providing a high temperature and the process will be allowed to continue without any alarm. Variables are: the length/diameter of the closed end tube, the orientation to the horizontal of the closed end tube, the insertion depth of the temperature sensor, the temperature setpoint to cause a pass/fail and the pressure at which the system senses the air detector temperature. Once the pass/fail decision has been taken, the air detector may remain active and some control systems may compare the sensor with the chamber drain throughout the sterilization stage.

The pressure controlled version is a similar closed ended pipe or chamber fitted to the autoclave chamber or drain. The closed pipe is capable of being isolated from the drain by means of an automatically controlled valve. The closed pipe/chamber is usually surrounded by a jacket or cooling coil, to which a cooling water supply can be connected/isolated. Instead of a temperature sensor the tube will have an accurate pressure transducer/pressure switch that operates in the vacuum range. The device will isolate a sample of steam by closing the isolation valve when the chamber reaches a pre-set pressure and applying cooling water to cause the contained steam sample to be condensed. The pass/fail decision will be made on the basis of the residual vacuum measured. In the event that a small (low) vacuum is measured, this is indicative of the presence of residual air and the process will caused to be failed. In the event that a deep vacuum is measured, it is indicative of little air, else its absence and the process will be allowed to continue. Variables are: the length/diameter of the closed end tube, the pressure setpoint to cause a pass/fail and the chamber pressure at which the system senses the air detector pressure.

Other common systems measure the autoclave drain temperature and chamber pressure and compare these to steam tables. It will be seen that by applying Dalton’s Law, that because of possible calibration errors of the pressure and temperature sensors, the sensitivity of the device will be restricted to sensing residual air volumes in the chamber of between 1 and 10%. Given that the amount of residual air at the end of an effective air removal process will be a number of orders lower than this, it will be seen that this approach will not detect leaks other than gross and it is unlikely that such devices will pass the requirements of EN 285//HTM 2010 (replaced by CFPP 01-01).

Setting up and Validating the Air Detector

A detailed explanation of how an air detector is set up can be found in HTM 2010 Part 3 (replaced by CFPP 01-01). Our experience has been that the process is either straightforward allowing the set up and testing to be completed in a relatively small number of test cycles, else will be a protracted exercise that can take weeks.

The device is testing using two tests, the performance and function tests.

Performance testing

The performance tests are used to demonstrate that the air detector will cause a cycle to be failed before the sterilization is deemed to have been affected.

After the air detector is set up, it is tested using the small load as described in EN 285/HTM 2010 (replaced by CFPP 01-01), and usually, in the pharmaceutical industry, a full load comprising of the EN 285 small load together with the production load having the greatest mass. Simplistically, in both cases, it is expected that a process will fail with an induced leak of 10 mBar/minute or less and the temperature depression, measured in the centre of the test pack will be less than 2o C when a thermocouple located in the chamber drain reaches the nominal sterilization temperature (typically 121o C). That is to say, when the drain thermocouple reaches 121o C, a thermocouple the centre of the test pack will read 119o C or more. The air detector will have to be disabled to prevent the cycle automatically failing. The test is deemed to have passed if the value measured on the air detector device would have caused a cycle to fail and the depression is less than 2o C.

The small and full load performance tests are carried out following set up during the initial validation, and annually thereafter.

Function testing

Because the air detector will not normally cause cycle failure, the function test is carried out to provide assurance that the device remains operational and will cause a cycle to fail when presented with the conditions established during the performance tests above.

A cycle is run a small load and with an induced leak of a value established during the performance tests described above. The test is considered satisfactory if the air detector causes the cycle to automatically fail.

The function test is carried out at a weekly or other relatively short frequency.